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Oscilloscopes, 5th Edition by Ian Hickman

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7
Digital storage
oscilloscopes
Since they were first introduced in 1971, the design and
performance of digital storage oscilloscopes - DSOs - has
advanced immeasurably. Furthermore, the pace of development
has quickened perceptibly in recent years. So in the fourth
edition of this book a chapter was devoted entirely to them.
However, even so it is only possible to cover their design and uses
in a fairly brief way. Readers requiring a book covering the
subject in greater depth should consult
Digital Storage Oscilloscopes,
Butterworth-Heinemann, ISBN 0 7506 2856 1, by the same
author.
The circuitry of conventional real-time oscilloscopes - the 'how
they work' - together with the construction of the c.r.t.s they use
are covered in Chapters 9 to 11, but in this chapter I have
followed the same plan as the preceding one on sampling scopes:
details on the measurement methods available with DSOs and on
how they are implemented by the internal circuitry of the scope
are all covered in one chapter. Figure 7.1, then, is a simplified
block diagram of a basic DSO. Comparing it with the block
diagram of a real-time analogue scope, see Figure 2.1, will show
considerable similarities: the major difference is that the vertical
signal, after passing through the input attenuator, Y preamplifier
and trigger pick-off stage, is not routed directly to the Y deflection
stage. Instead it is sampled at intervals and the samples fed to an
ADC to be 'digitized', i.e. converted to a string of numbers: each
number represents the voltage of the input signal at the instant
the corresponding sample was taken. The digitized data is stored
in a 'channel store', i.e. that part of the total digital memory
which is allocated to the particular Y input channel, of which
there are usually at least two and often more. The digital memory
consists of a bank of RAM (random access memory) ICs
(integrated circuits).
116 Oscilloscopes
I
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and control amphfier
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Figure 7.1 Simplified outline block diagram of a [ypical DSO (digital storage
oscilloscope)
For display purposes, the data currently in the store is read out
sequentially and the samples passed to a DAC - a digital-to-
analogue converter. There they are reconstituted into a series of
discrete voltage levels forming a stepwise approximation to the
original waveform. This is fed, along with the reconstituted
waveforms(s) from the other Y channel(s), to the vertical
deflection amplifier for the usual dual or four trace display. Note
that the readout and display of samples constituting the stored
waveform need not occur at the same sample rate that was used
to 'acquire' the waveform in the first place. It is sufficient to use
a display sample rate adequate to ensure that each and every
trace displayed is rewritten fifty or more times a second; this will
prevent flicker of the display. This means that in principle, as we
saw with sampling scopes in Chapter 6, one could use a Y
deflection amplifier and c.r.t. (or LCD display panel) with very
modest bandwidth as the display in a DSO, even though the
instrument as a whole is capable of displaying signals with a
bandwidth of tens or even hundreds of megahertz. In practice,

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